The present invention relates to a process for identifying statistically homogeneous pixels in SAR images acquired on the same area.
As is known, a synthetic aperture radar (or SAR) system produces a two-dimensional image. One dimension of the image is called the range and is the measurement of the distance in a view line from the radar of the object that is being illuminated. The other dimension is called the azimuth and is perpendicular to the range.
The SAR radar operates at a frequency that is generally comprised between 400 Mhz and 10 Ghz, and is usually installed on aeroplanes or satellite platforms orbiting at a height of between 250 and 800 Km. The antenna of the radar points to earth orthogonally to the direction of motion of the platform (aeroplane or satellite) at an angle known as the “offnadir” angle comprised between 20 and 80 degrees compared with the nadir direction, i.e. perpendicularly to the earth.
With this system images of the earth's surface can be generated with a spatial resolution of a few meters, thus synthesising (hence the name of the sensor) in the direction of the azimuth an antenna of much greater dimensions than real dimensions by means of appropriate algorithms that are well known in the literature.
The most important feature of the SAR is that it is a consistent sensor and therefore the images are matrices of complex numbers the amplitude values of which are linked to the power that is backscattered by the illuminated objects (i.e. to the radar cross section thereof), whilst the step is determined by the nature of the target and the distance of the target from the radar. Owing to the possibility of obtaining images regardless of sunlight conditions and cloud cover, the SAR images are suitable for different applications; amongst these, the applications linked to the identification and classification of targets, “change detection” and interferometry applications are of primary importance. The latter are usually aimed at obtaining digital elevation models and/or the analysis of surface deformation of the terrain from sets of multitemporal SAR data.
Often, in the processes used for extracting information of interest from radar data, it is useful to group together homogenous pixels of the images that are statistically homogenous, i.e. the radar signature of which is substantially identical. Let it suffice to think of the problems connected with filtering (denoising) data: sound reduction, whether it be of amplitude or of phase, is normally conducted to the detriment of the spatial resolution of the data. In practical terms, in fact, denoising is nearly always translated into a movable average operation conducted on an estimation window around each image pixel. A similar approach is also used to estimate other parameters of interest linked, for example, to geophysical or electromagnetic properties of the illuminated targets, such as temporal consistency, the scattering matrix in polarmetric data, etc.
A general SAR image gathers data coming from targets of various natures: natural (woods, rocks, meadows, etc) or artificial (manufactured products, metal structures, motor vehicles, etc). The electromagnetic features can thus vary significantly even between adjacent pixels of a radar image. There can also be distributed targets that generate almost identical radar features of the radar signal, for an entire group of pixels, both punctiform or characterising a single pixel or at most a few other pixels connected thereto. These two types of target normally require processing that is substantially different in current applications. For example, adopting filtering techniques that prove to be useful for the first type of target (distributed) is harmful for the second (punctiform) that are typically characterised by high values of the signal-noise ratio (SNR), and where the spatial average operation conducted on adjacent pixels, characterised by lower SNRs, is counterproductive.
In view of the prior art, the object of the present invention is to provide a process for identifying statistically homogeneous pixels of SAR images acquired on the same area, i.e. characterised by similar electromagnetic properties, on which it is possible to make reliable statistical estimates, on a support selected in an adaptive manner.
According to the present invention, this object is achieved by a process for identifying statistically homogeneous pixels in images acquired on the same area by means of a synthetic aperture radar (SAR sensor) comprising the following steps:
(a) acquiring a plurality of N radar images by means of a SAR sensor on the same area with acquisition geometries that are suitable to consent the common grid resampling of data,
(b) after the common grid resampling, selecting a pixel and identifying the pixel as a sample pixel,
(c) calculating a vector of N amplitude or intensity values relative to the sample pixel in the N available images and identifying the vector as a sample vector,
(d) defining an estimation window for the sample pixel to identify a set of pixels in the neighbourhood of the sample pixel,
(e) calculating the vectors of N amplitude or intensity values for every pixel contained in the estimation window, similarly to what has been done for the sample pixel;
(f) comparing, by means of a statistical test, every vector of amplitude or intensity values of the pixels belonging to the estimation window with the sample vector to verify which of the vectors of amplitude or intensity values are generated by the same probability distribution function of the sample vector,
(g) identifying those pixels associated with the vectors of amplitude or intensity values that have passed the test (f) as being pixels that are homogenous with the sample pixel and identifying as pixels that are not homogeneous with the sample pixel those pixels associated with the vectors of amplitude or intensity values that have not passed the test (f),
(h) eliminating the pixels contained in the estimation window that, even if associated to vectors that have passed the test (f), yet do not result connected to the sample pixel, directly or by means of other pixels that resulted homogeneous to it,
(i) identifying the pixels that are homogeneous and connected to the sample pixel as being statistically homogeneous pixels.
Owing to the present invention it is possible to implement a process for identifying statistically homogeneous pixels of SAR images acquired on the same area on which to conduct general processing, thus drastically reducing the problems generated by averages and estimates made on statistical populations of strongly inhomogeneous data that can lead to the filtering results, and more in general, the analysis being completely distorted.
In addition, the process according to the invention also enables the response of the punctiform radar targets to be preserved.